In chemical process plants power plants and other similar environments pneumatic pressures frequently are supplied at various points in the system to operate control instruments, such as valve positioners or actuators. Often it is desirable to effectuate the pressure changes required for achieving control functions (e.g., changing valve stem positioner settings, and the like) from a remote, central location, such as a control room. This control room may be located as much as 1,000 feet or more (by cable) from the device to be controlled. It is customary to employ an electrical current (from a controllable current source) as the variable for effecting the pressure changes, due to the need to operate over such large distances. Typically, a series current loop is established between the control room and the remote location, and the load or device to be controlled (i.e., the control instrument) is placed in the series loop.
Frequently, hazardous atmospheres may exist in the vicinity of the controlled device. Therefore, very low energy electricity is employed to obtain a condition of Intrinsic Safety--that is, to ensure that there is insufficient energy stored in any element such that, in the event of a spark, ignition of the hazardous atmosphere is prevented. To achieve this objective, there is typically a maximum of about ten volts available to the terminals of the load, at a current which may vary from about 3.2 mA to about 20 mA
Presently available current-to-pressure transducers which are designed to operate in such environments, have several drawbacks. They are not very accurate or stable; they are sensitive to temperature changes and vibration and will cause the output pressure to fluctuate under vibratory stimuli; they have a limited dynamic range and loop gain; they are not easy to align or balance; and they have components that degrade due to wear. In addition, they are relatively large and often require complex manufacturing and assembly techniques and equipment; this makes them expensive to produce. If explosion proof housings should be needed, they too, add to the cost of the transducers.
One relatively new current-to-pressure converter which overcomes some of the drawbacks of the aforementioned current-to-pressure transducers is illustrated in the November 1981 issue to the Spanish magazine "Regulacion y Mando Automatico" in an article entitled "Nuevo Principio de Funcionamiento Para Convertidores I/P". The current-to-pressure transducer described in the article includes an electronic control loop employing a PID (Proportional Integral Derivative) converter. A small electrical input signal to the converter is connected to the PID converter. The signal is compared with one measured by a solid state transducer. If a difference exists, a voltage is applied to a piezo-electric element covering an air nozzle. The movement of the element alters the air pressure at the nozzle to in turn operate a pneumatic relay coupled thereto. The operation of the relay adjusts the output signal to match the input signal, thereby completing the control function. This current-to-pressure transducer is patented by the Saab-Scania Company under Swedish Patent No. 7901841 and Germant Patent DE No. 30 34 326. It is also the subject of a PCT (Patent Cooperation Treaty) application (See Great Britain application No. 2,065,331) in several other countries.
While the Saab-Scania current-to-pressure transducer appears to be an improvement over the current-to-pressure transducers previously described, it too suffers certain drawbacks. For one, the use of the PID controller makes the device more complex. Furthermore, the device is not designed to meet the low power Intrinsic Safety requirements that are often necessary when using a current-to-pressure transducer in a hazardous atmosphere or environment.